1. Field of the Invention
This invention relates to an apparatus for reading out a radiation image stored in a stimulable phosphor sheet. This invention particularly relates to a radiation image readout apparatus which quickly conducts image read-out even when stimulating ray sources of small outputs are used for stimulating the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the radiation energy stored. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy.
FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed for reading out a radiation image stored in a stimulable phosphor sheet in the aforesaid radiation image recording and reproducing system.
In the apparatus of FIG. 1, a laser beam 1a of a predetermined intensity is emitted as stimulating rays by a laser beam source 1 to a galvanometer mirror 2. The laser beam 1a is deflected by the galvanometer mirror 2 to form a laser beam 1b impinging upon a stimulable phosphor sheet 3 positioned below the galvanometer mirror 2 so that the sheet 3 is scanned by the laser beam 1b in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam 1b impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. There for, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1b. As the stimulable phosphor sheet 3 is scanned by the laser beam 1b, the portion of the sheet 3 exposed to the laser beam 1b emits light having an intensity proportional to the radiation energy stored. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its light input face 4a positioned close to the sheet 3 in parallel to the main scanning line. The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its light input face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or stored in a magnetic tape 8, or directly reproduced as a hard copy on a photographic material or the like.
In order to make the aforesaid radiation image read-out apparatus small, the stimulating ray source should be made small. As the stimulating ray source satisfying this need, a laser diode (hereinafter abbreviated as LD) is suitable. However, in general, the output of LDs is small, and the sensitivity of the stimulable phosphor sheet to the long-wavelength light which is the principal part of the light that the LD emits is markedly low. Therefore, when an LD is used as the stimulating ray source, image read-out cannot be achieved sufficiently or the read-out speed becomes low.
The aforesaid problem can be solved by using a plurality of LDs and combining the laser beams emitted thereby by substantially merging or coalescing the beams into a single laser beam to obtain stimulating rays of high-intensity. However, it is technically difficult to combining the optical axes of many laser beams deflected by light deflectors. The read-out speed can also be increased by combining the LDs with light deflectors and light guide members to form a plurality of scanning systems and scanning different portions of the stimulable phosphor sheet simultaneously by the scanning systems. However, when a plurality of scanning systems are used, the read-out apparatus becomes large and the aforesaid aim of making the apparatus small cannot be accomplished.